5_seismology

GMT for Seismology: Plotting beachballs

Instructor: Dongdong Tian

GMT provides 4 seismology modules (see the summary figure below). The goal of this session is to show how to plot beachballs on maps and cross-sections.

Contents

• Explain Focal Mechanism
• Plot beachballs on maps
• Plot beachballs on cross-sections
• Get focal mechanism solutions from Global CMT
• Group Exercise

Resources

• Focal Mechanisms Explained: What are those “beach balls”? https://www.youtube.com/watch?v=MomVOkyDdLo (about 6 minutes)
• manpage of meca
• manpage of coupe
• Global CMT project

Focal mechanism and beachball

Focal mechanism describes the movement of the fault that generates the seismic waves. From focal mechanism solutions, we can know the orientation of the fault plane and the slip direction.

Beachball is a graphic represention of focal mechanism. Here are three simple focal mechanism solutions for normal fault, reverse fault, and strike-slip fault.

We will use these three focal mechanisms as the example data.

# longitude latitude depth strike dip rake magnitude [event_title]
112 32 25  30  90   0  4  Strike-slip
115 34 15  30  60  90  5  Reverse
118 32 45  30  60 -90  6  Normal

First, we need to copy the data and save it into a plaintext file meca.dat, which will be used in the following scripts.

Plot beachballs on maps

The meca module can plot beachballs on maps.

-S is the most important option of meca. It tells GMT the format of the input data, beachball size, and other properties of optional title string.

Other useful options:

• -E: color for extensive quadrants
• -G: color for compressional quadrants
• -C: paint the compressional quadrants based on depth (you need to create a CPT by calling makecpt module first)

The simplest example: beachball.sh

Beachballs with the same size: beachball-same-size.sh

Beachballs with customized event labels: beachball-customized-labels.sh

Beachballs without event labels: beachball-no-labels.sh

Different colors: beachball-colors.sh

Color by depth: beachball-color-by-depth.sh

Plot beachballs on cross-sections

coupe module can cut and plot beachballs in cross-sections (e.g., a vertical cross-section).

Most options of coupe have the same meaning as in meca. The most important and unique option is -A, to determine the cross-section.

There are different ways to specify the geometry of a cross-section. Here we will use -Aa as an example. The full syntax of the -A option is:

-Aalon1/lat1/lon2/lat2[+ddip][+wwidth][+zdmin/dmax][+r]

• lon1/lat1: location of the starting point
• lon2/lat2: location of the end point
• dip: dip angle of the cross-section: 0 for horizontal slice, 90 for vertical slice
• width: width of the cross-section
• dmin/dmax: depth range of the cross-section
• +r: automatically determine the plot region! -R is not necessary!

Cross-section view: beachball-cross-section.sh

Get focal mechanism solutions from Global CMT

Global CMT: https://www.globalcmt.org/

The GCMT project provides focal mechasnism solutions for global earthquakes with M>5 since 1976.

1. Go to the GCMT official site, and click the CMT catalog web search link.

2. Enter the parameters to select earthquakes:

   • start time and end time
   • magnitude range
   • location
   • etc

3. Select "Output type": "GMT psmeca input" (NOT "GMT psvelomeca input")

4. Click the "Done" button

5. Copy/paste the solutions into a plaintext file and save it.

6. Now you can plot the beachballs using meca or coupe

NOTE: Use -Sd, -Sm or -Sz for GCMT solutions, NOT -Sa.

Group Exercise

You will be split into teams to work on an exercise:

• Discuss with your team which commands and options you would use
• Work together to make a script that generates the desired plot
• If you have any questions, ask on the Slack chatroom

Make a regional map with beachballs colored by depth

• Select the region you want to work on
• Go to GCMT web search site, select the focal mechanism solutions by specifying geographic range, time range, and magnitude range. Save the solutions to a file.
• Make a map view of beachballs with colors determined by depth
• Based on a the map view plot, choose a cross-section, and make a cross-section view

TIPS

• You need to use -Sd, -Sm or -Sz for GCMT solutions, NOT -Sa.
• Use +f0p in the -S option to set the font size of the event labels to zero, so that meca and coupe won't plot the event lables
• You may have white beachballs if the event depth is out of the CPT range

Map view (meca)	Cross-section view (coupe)

SOLUTION

Don't look at the solutions until you finish you own script.

The examples above are plotted using data japan-focal.dat using script japan-beachballs.sh.